Compressible films surrounding solder connectors

ABSTRACT

Disclosed is a method of forming an integrated circuit structure that forms lead-free connectors on a device, surrounds the lead-free connectors with a compressible film, connects the device to a carrier (the lead-free connectors electrically connect the device to the carrier), and fills the gaps between the carrier and the device with an insulating underfill.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No. 10/711,076filed Aug. 20, 2004, the complete disclosure of which, in its entirety,is herein incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention generally relates to connectors between devices andcarriers and more particularly to connectors that are surrounded bycompressible material that prevents delamination of the carrier from thedevice.

2. Description of the Related Art

Devices such as integrated circuit chips are often connected to carriersthat include wiring connections to the integrated circuit chips. Theintegrated circuit chips can be connected to the carriers using aconductive lead solder. These lead connectors are generally formed asballs on the carrier and/or the chip. The carrier and chip are generallyheated to cause the solder to melt, after which the structures areallowed to cool so the solder solidifies. This process is described as a“reflow” process and it bonds the lead solder connection to both thecarrier and the chip.

Often, an insulating underfill material is used to fill in the remainingspace between the device and the carrier. This underfill helps increasefatigue life of solder interconnections by absorbing some of the stressthat results from the difference in the coefficients of thermalexpansion of semiconductor devices and ceramic or organic carriers.

Though lead-containing solders have been used for decades and exhibithigh yield and reliability due to their extensive utilization, worldwidelegislation and environmental concerns have led to considerable interestin the development and use of lead-free solders. One such lead-freesolder is SnAgCu, commonly called SAC, which is one of the leadingalloys being considered as an alternative to solder connectionscontaining lead. The SAC alloy (available with various levels of Ag andCu, but typically ranging from 3-4% Ag and 0.5-1% Cu) has numerousadvantages including a relatively low melting point, good fatigue life,and compatibility with common lead-free solders. Consequently, SAC isone of the leading candidates for lead-free interconnects betweensemiconductor devices and chip carriers.

One of the drawbacks in using lead-free solders is that their majorconstituents tend to experience a relatively large (e.g., 3%) volumeexpansion upon reflow. Unfortunately, the volume expansion of lead-freesolders can force the underfill away from the solder connection, whichprevents the underfill from being able to maintain support of the solderwhen the solder cools back to its original volume. As a result, thislarge volume expansion upon reflow prevents some lead-free solders frombeing used on ceramic or organic carriers that require underfill.

SUMMARY OF THE INVENTION

Disclosed is a method of forming an integrated circuit structure, wherethe method forms lead-free connectors on a device, surrounds thelead-free connectors with a compressible film, connects the device to acarrier (the lead-free connectors electrically connect the device to thecarrier), and fills the gaps between the carrier and the device with aninsulating underfill.

The connectors can be reflowed by heating to melting, and then cooling.Some features of the embodiments herein are that the compressible filmhas a melting point above the lead-free connectors and the compressiblefilm has sufficient compressibility to accommodate expansion of thelead-free connections when the lead-free connections are melted withoutdamaging the underfill. Also, the process of surrounding the lead-freeconnectors with the compressible film can form the compressible filminto a pattern between the carrier and the device where the compressiblefilm is positioned around less than all the lead-free connections. Thispattern can, for example, form channels between the device and thecarrier, wherein the channels are filled with the underfill, or thepattern can comprise diagonal stripes of the compressible film.

The resulting structure has the device connected to the carrier bylead-free connectors with the compressible film surrounding (orpartially surrounding) the lead-free connectors, and the insulatingunderfill filling gaps between the carrier and the device.

These, and other, aspects of the embodiments herein will be betterappreciated and understood when considered in conjunction with thefollowing description and the accompanying drawings. It should beunderstood, however, that the following description, while indicatingembodiments of the present invention and numerous specific detailsthereof, is given by way of illustration and not of limitation. Manychanges and modifications may be made within the scope of the presentinvention without departing from the spirit thereof, and the inventionincludes all such modifications.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be better understood from the following detaileddescription with reference to the drawings, in which:

FIG. 1 is a schematic cross-sectional diagram of a partially completeddevice and carrier structure;

FIG. 2 is a schematic cross-sectional diagram of a partially completeddevice and carrier structure;

FIG. 3 is a schematic cross-sectional diagram of a partially completeddevice and carrier structure;

FIG. 4 is a schematic cross-sectional diagram of a partially completeddevice and carrier structure;

FIG. 5 is a schematic cross-sectional diagram of a partially completeddevice and carrier structure;

FIG. 6 is a schematic cross-sectional diagram of a partially completeddevice and carrier structure;

FIG. 7 is a schematic cross-sectional diagram of a connection structure;

FIG. 8 is a schematic top-view diagram of one pattern of compressiblematerial;

FIG. 9 is a schematic top-view diagram of one pattern of compressiblematerial;

FIG. 10 is a schematic top-view diagram of one pattern of compressiblematerial;

FIG. 11 is a graph showing the relationship between the reduction frommaximum pressure verses the compressive modulus for differentthicknesses of compressible material; and

FIG. 12 is a flow diagram illustrating one embodiment.

DETAILED DESCRIPTION

The present invention and the various features and advantageous detailsthereof are explained more fully with reference to the nonlimitingembodiments that are illustrated in the accompanying drawings anddetailed in the following description. It should be noted that thefeatures illustrated in the drawings are not necessarily drawn to scale.Descriptions of well-known components and processing techniques areomitted so as to not unnecessarily obscure the present invention. Theexamples used herein are intended merely to facilitate an understandingof ways in which the invention may be practiced and to further enablethose of skill in the art to practice the invention. Accordingly, theexamples should not be construed as limiting the scope of the invention.

The various embodiments herein use a compressible film around the deviceto carrier connection to provide a volume into which the lead-freesolder (e.g., SAC alloy) can expand (before it reaches the underfill),thereby allowing the underfill to support the “bumps” even afternumerous thermal excursions. The result is that lead-free solders can beused with all their advantages, without incurring the negative impact oflead-free solder volume expansion.

More specifically, as shown in FIG. 1, one embodiment forms lead-freeconnectors 12 on a device 10. Item 10 can comprise any type of devicethat is to be connected to any type of carrier. For example, item 10could comprise an integrated circuit chip having functional devicestherein. Alternatively, item 10 can represent the carrier, if theconnectors 12 are formed on the carrier first. The connectors 12 are anytype of electrically conductive connector that suffers from volumeexpansion upon reflow. For example, the connectors 12 can comprise alead-free solder, such as the SAC alloy that is discussed above.

As shown in FIG. 2, in this embodiment, a compressible material 20 isformed to surround sides of the connectors 12. In one example, thecompressible material 20 can be deposited at the level shown in FIG. 2.Alternatively, additional compressible material can be deposited and thestructure can be planarized down to the level shown in FIG. 2. Onefeature is that the top of the connector 12 is exposed such that it canform an electrical connection to the carrier when it is attached to thecarrier.

The compressible material 20 can be any compressible material, such ascompressible silicone rubber, polyimide foam, or any other material thatis thermally stable above the melting point of the connectors 12 (e.g.,260° C.) and has an expansion coefficient in the desired range for theexpansion of each connector 12. One feature is that the compressiblefilm 20 has sufficient compressibility to accommodate expansion of theconnectors 12 when the connections are melted without damaging theunderfill.

In FIG. 3, a mask 30 is formed using photolithographic techniques and,in FIG. 4, the compressible material 20 is patterned. This patterningprocess can be any conventional material removal process, such asetching, laser processing, and other similar methodologies. It is alsopossible that a photosensitive or electron-beam-sensitive compressiblematerial 20 could be used, which would avoid the need to use the mask30. Some of the possible patterns that the compressible material 20 canbe patterned into are shown in FIGS. 8-10, and are discussed in greaterdetail below.

In FIGS. 1-4, the compressible material 20 is formed before theconnectors 12 are reflowed into spheres. However, the compressiblematerial 20 could be applied after the connectors 12 are reflowed intospheres, as shown in FIG. 5. Also note that FIG. 5 illustrates that insome embodiments, the compressible material 20 does not need to come upto the top of the connector 12. Instead, certain designs may see benefitfrom only using the compressible material 20 along a portion of thesides of the connectors 12.

After the connectors 12 are surrounded with the compressible film 20,the device is connected to a carrier 60, as shown in FIG. 6. Theconnectors 12 electrically connect the device 10 to the carrier 60.Then, an insulating underfill 62 is deposited to fill the gap betweenthe carrier 60 and the device 10. In the resulting structure, the device10 is connected to the carrier 60 by connectors 12 with the compressiblefilm 20 surrounding (or partially surrounding) the connectors 12, andthe insulating underfill 62 filling gaps between the carrier 60 and thedevice 10.

FIG. 7 illustrates one of the connectors 12, the surroundingcompressible film 20 and a portion of the underfill 62 in cross-section.FIG. 7 illustrates that the connector 12 can expand when heated and howit may expand permanently after being heated (reflowed). Note that FIG.7 is not drawn to scale. In FIG. 7, line 74 illustrates the size of theconnector 12 before heating, line 70 represents the size of theconnector 12 when it is in a liquid or molten state (while it is beingheated), and line 72 represents the size of the connector 12 after areflow process. The compressible material 20 compresses to accommodatethe change in size of the connector 12, which avoids deforming theunderfill 62. This allows the underfill 62 to remain connected to thecompressible material 20, the device 10, and the carrier 60 regardlessof the expansion of the connector 12. Further, the compressible material20 will allow further accommodation of volume expansion during otherdownstream high temperature processes (card assembly, rework, etc.) aswell. With the inventive use of the compressible material 20, theunderfill 62 will be able to provide the structural coupling requiredbetween the device and carrier.

As shown in FIG. 6, the compressible film 20 is patterned between thecarrier 60 and the device 10. The patterns shown in FIG. 8-10 allow theunderfill 62 to more easily fill all spaces between the carrier 60 andthe device 10. Note that FIGS. 8-10 illustrate some exemplary patternsof the compressible material 20 and the invention is not strictlylimited to these examples. In FIG. 8, the compressible material 20 formsa film around the ball or sphere shaped conductor connector 12. In FIG.9, the compressible material 20 is patterned into squares or rectangles,thereby forming channels 90 through which the underfill 62 can beinjected. This pattern can, for example, comprise diagonal stripes ofthe compressible film, as shown in FIG. 10. One feature of the patternof the compressible material 20 is that it leaves sufficient room forthe underfill material 62 to provide a good structural bond between thedevice 10 and the carrier 60. Note that while these examples show thatall the connectors 12 have compressible film 20 thereon, in otherembodiments, the compressible film 20 is positioned around less than allthe connectors 12.

The makeup of the compressible film 20 that is used will vary dependingupon the amount of compression that is needed in a specific design. Inaddition, the thickness of the compressible material 20 will vary,again, depending upon the amount of compression required. FIG. 11 is agraph showing the relationship between the reduction from maximumpressure verses the compressive modulus for different thicknesses ofdifferent compressible materials. The left side of the graph illustratesthe range of elastomers while the center of the graph illustrates therange of polyimides, polysulfone, nylon, etc. In addition, the curvesrepresent different thicknesses of the compressible material (e.g., 5,6, and 7 mil). These curves represent that, as the compressible materialis made thinner, it must be softer to provide the same degree ofpressure reduction.

As mentioned above, the compressible material does not need tocompletely surround the sides of the connector 12. Covering only part ofthe ball height has significant advantages for flux removal and tofacilitate underfill flow to narrow channels. However, pressure buildupwill be greater as the entire connector ball 12 expands, but only aportion of its height can accommodate the extra volume. For example, ifthe compressible material comes up two-thirds of the ball height, thepressure would be 50% higher than if the compressed material covered theentire side of the connector 12. Thus, the specific material andthickness used will vary depending upon each design, as will the amountof the side of the connector that is covered.

FIG. 12 shows an embodiment in flowchart form. In FIG. 12, item 120 ofthis embodiment forms connectors that may expand on a device. In item122, this embodiment surrounds the connectors with a compressible film.Next, in item 124, this embodiment connects the device to a carrier (theconnectors electrically connect the device to the carrier). In item 126,this embodiment fills the gaps between the carrier and the device withan insulating underfill.

As shown above, various embodiments herein use a compressible filmaround the device to carrier connector to provide a volume into whichthe connector can expand (before it reaches the underfill), therebyallowing the underfill to support the device and carrier, even afternumerous thermal excursions. The result is that lead-free solders (andother connector materials that suffer unwanted expansion upon heating)can be used with all their advantages, without incurring the negativeimpact of connector volume expansion.

While the invention has been described in terms of preferredembodiments, those skilled in the art will recognize that the inventioncan be practiced with modification within the spirit and scope of theappended claims.

1. A method of forming an integrated circuit structure, said method comprising: forming solder connectors on a device; surrounding sides of said solder connectors with a compressible film; connecting said device to a carrier, wherein said solder connectors electrically connect said device to said carrier; and filling gaps between said carrier and said device with an insulating material.
 2. The method in claim 1, further comprising melting said solder connectors, wherein said compressible film is stable above the melting point of said solder connectors.
 3. The method in claim 1, further comprising melting said solder connectors, wherein said compressible film has sufficient compressibility to accommodate expansion of said solder connections when said solder connections are melted without damaging said insulating material.
 4. The method in claim 1, wherein said process of surrounding sides of said solder connectors with said compressible film forms said compressible film into a pattern between said carrier and said device.
 5. The method in claim 4, wherein said pattern has channels between said device and said carrier, wherein said channels are filled with said insulating material.
 6. The method in claim 4, wherein said pattern comprises diagonal stripes of said compressible film.
 7. The method in claim 1, wherein said process of surrounding sides of said solder connectors with said compressible film positions said compressible film partially up sides of said solder connections.
 8. A method of forming an integrated circuit structure, said method comprising: forming lead-free connectors on a device; surrounding sides of said lead-free connectors with a compressible film; connecting said device to a carrier, wherein said lead-free connectors electrically connect said device to said carrier; and filling gaps between said carrier and said device with an insulating underfill.
 9. The method in claim 8, further comprising melting said solder connectors, wherein said compressible film is stable above the melting point of said solder connectors.
 10. The method in claim 8, further comprising melting said lead-free connectors, wherein said compressible film has sufficient compressibility to accommodate expansion of said lead-free connections when said lead-free connections are melted without damaging said underfill.
 11. The method in claim 8, wherein said process of surrounding sides of said lead-free connectors with said compressible film forms said compressible film into a pattern between said carrier and said device.
 12. The method in claim 11, wherein said pattern has channels between said device and said carrier, wherein said channels are filled with said underfill.
 13. The method in claim 11, wherein said pattern comprises diagonal stripes of said compressible film.
 14. The method in claim 8, wherein said process of surrounding sides of said solder connectors with said compressible film positions said compressible film partially up sides of said solder connections. 